1. Field of the Invention
The invention relates to a developing unit used for an image forming apparatus such as a printer, a facsimile machine and a copying machine using an electro-photographic process.
2. Description of Related Art
In general, an image forming apparatus such as a printer using an electro-photographic process includes a developing unit configured to develop images by carrying toners as developers to photosensitive drums as electrostatic latent image carriers. The developing unit includes: toner conveying rollers for conveying toner; and developing rollers for conveying the toner, conveyed by the toner conveying rollers, to the photosensitive drum. Each of the developing rollers is in contact with the toner conveying roller on one side, and is in contact with the photosensitive drum on another side. An electrostatic latent image is formed on the photosensitive drum by exposing the photosensitive drum with light. The toner on the surface of the developing roller is attracted to the electrostatic latent image, thereby rendering the electrostatic latent image visible.
A developing blade is in press-contact with the surface of the developing roller. The developing blade is configured to triboelectrically charge toner, and concurrently to restrict the thickness of the toner layer on the developing roller. The toner is negatively charged triboelectrically by this developing blade. The surface of the photosensitive drum is initially uniformly negatively charged, and then an electrostatic latent image based on image information is formed on the surface by an LED light beam or a laser light beam. The negatively-charged toner is attracted to this electrostatic latent image area. Thereby, the image is developed.
The developing unit has the following problems. The surface of the photosensitive drum is charged unevenly to a relatively large extent. In other words, the distribution of electrostatic charge is so wide that the toner often adheres to an area beyond the electrostatic latent image area when being attracted to the photosensitive drum. This causes what is termed as “sheet fog.”
To solve this problem, Japanese Patent Application Publication No. 2003-84558 discloses that a developing unit includes a developing roller, a toner conveying roller, a developing blade, a toner charging roller, a secondary toner conveying roller and a secondary developing blade. In addition, according to Japanese Patent Application Publication No. 2003-84558, the apparatus is capable of triboelectrically charging toners sufficiently enough to reach an almost saturated level, and thereby minimizing unevenness of the electrostatic charge. The patent application discloses that the apparatus is capable of reducing the likelihood that the sheet fog may occur during developing.